The reduction of small molecules, such as nitrogen and carbon dioxide, is extraordinarily difficult because the one-electron reduction processes often involve high-energy intermediates. For example, in the fixation of nitrogen (conversion of N2 to NH3), the reaction N2+e−→N2− involves such high energy that the gas-phase anion N2− exists only as a fleeting transient. The standard reduction potential for the analogous solution-phase reaction N2+e−→N2−(aq) has been estimated at −4.2 V vs. the normal hydrogen electrode (NHE). While some evidence exists for formation of N2− species at surfaces of ionic oxides such as MgO, nitrogen reduction is usually accomplished by coupling with the transfer of one more protons. Yet, even these have high energy; the reaction N2+H++e−→N2H, has a calculated reduction potential E° of −3.2 V vs. the normal hydrogen electrode (NHE).
The photocatalytic reduction of nitrogen was first discovered by Schrauzer and Guth (G. N. Schrauzer, T. D. Guth, Journal of the American Chemical Society 99, 7189 (1977)), who showed that N2 could be reduced to NH3 on the surface of TiO2 powder when illuminated with light from a mercury arc lamp. Although since then various modified TiO2 catalysts have been developed, the overall efficiency of the reaction remains poor. The poor efficiency arises because the proton-coupled reactions have relatively complicated pathways, and because the highly stable N2 molecule has only a very low binding affinity for surfaces.